1. Field of the Invention
The present invention is related to a filtration system and, more particularly, to a method and apparatus for controlling the operation of gravity or pressure filters for the removal of suspended material therefrom in water and waste water treatment plants.
2. Description of the Prior Art
Water and waste water treatment processes include flow sheets which incorporate pre-treatment steps which involve, in the case of water treatment, coagulation, flocculation, and sedimentation. In cases involving waste water treatment, the pre-treatment phase may involve biological or chemical treatment or a combination thereof. In either event, the water or waste water from the preliminary pre-treatment processes contains carryover suspended material which must be removed prior to use. The process utilized for the removal of the carryover suspended material is known in the art as filtration. Filtration processes involve passing the water or waste water through a porous bed of granular material which is composed of graded layers of materials having varying sizes and densities. Filters have been constructed of, for example, silica sand, anthracite, garnet, illmenite, or other suitable material. Current practice is to construct such filters from a combination of different materials having varying particle size and density so that the material, when subjected to an upward flow of water during a backwash, will grade itself in reverse order with the largest diameter, lowest specific gravity material on the top of the bed and the finest and highest specific gravity material on the bottom. Such filters are further generally classified as being of the gravity type or the pressure type.
As the water passes through the filter, the suspended material is removed by virtue of its attachment to the grains of the filter material. Beginning with a relatively clean filter, very little resistance to flow is offered by the granular filter material. However, as the water continues to pass through the filter, the suspended material is gradually removed tending to fill the voids between the grains, thereby gradually increasing the flow resistance. After a certain period of operation, known as a run, the filter will have removed sufficient suspended material to fill up all available voids. At this point, very little flow will pass through the filter due to the high resistance created by the clogged filter bed. The filter is then backwashed by reversing the flow upwards through the filter, expanding the filter bed due to the upward velocity, and washing the entrapped suspended material away to drain. At the completion of a backwash operation, the relatively clean filter is placed back in service and the foregoing cycle is repeated.
Since the filter bed creates a varying resistance to flow during each cycle, it is generally necessary to provide a system for controlling the flow of water through the filter. One prior art technique, known as constant rate control, utilizes a flow sensing device and a flow throttling device installed in the effluent pipe from the filter. Control instrumentation is provided which compares a flow signal from the flow sensing device with another signal representing the desired flow. The output signal from the comparator-controller regulates the flow throttling device to bring the flow rate into balance with the desired or set rate. In such a device, the flow through the filter is controlled at a fixed or constant rate, independently of the effect of the variable resistance offered by the filter bed.
Another prior art technique, known in the art as constant level control, causes the filter to accept all water that flows thereto. In this technique, water enters the filter over a weir. When multiple filters are utilized, the influent weirs serve to divide the flow of water equally among all filters. A level sensing device installed in each filter provides an output signal which is utilized to regulate a throttling device installed in the effluent pipe from the filter so as to maintain the water level in the filter at a fixed point just below the level of the influent weir. In this technique, the water flow through the filter always equals the flow into the filter over the weir, regardless of the variable resistance offered by the filter bed.
Another prior art technique, known as the declining rate or variable declining rate method, is normally used with several filters operating in parallel in which a common influent and effluent header is provided for all filters. Pressure in the common influent header is raised or lowered to vary the flow through all filters. Influent connections into each filter are below the water surface to thereby create a common water level in all filters. The water level in the filters then assumes a level required to provide sufficient head pressure to produce the flow called for by the pressure in the common effluent header. A fixed restriction, such as an orifice, is built into the effluent pipe from each filter such that no filter, when clean after backwashing, will take an undue share of the total load. This technique, as exemplified by U.S. Pat. No. 3,771,655, is considered by those skilled in the art to be a very efficient method of control of the filtration process since it utilizes the cleanest filter to pass the most flow.